Transformer-based doherty power amplifier

ABSTRACT

Transformer-based Doherty power amplifier (PA). In some embodiments, a Doherty PA can include a carrier amplification path having an output that includes a carrier transformer, and a peaking amplification path having an output that includes a peaking transformer. The Doherty PA can further include a combiner configured to combine the outputs of the carrier and peaking amplification paths into an output node. The combiner can include a quarter-wave circuit implemented between the carrier and peaking transformers.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application Nos.62/028,018 filed Jul. 23, 2014, entitled TRANSFORMER-BASED DOHERTY CMOSPOWER AMPLIFIER, the disclosure of which is hereby expresslyincorporated by reference herein in its entirety.

BACKGROUND

1. Field

The present disclosure relates to Doherty power amplifiers.

2. Description of the Related Art

A Doherty power amplifier (PA) typically includes two amplificationpaths. The first amplification path typically includes a carrier PA, andthe second amplification path typically includes a peaking PA. An inputradio-frequency (RF) signal is typically split between the twoamplification paths. After the split RF signals are amplified in theirrespective amplification paths, they are combined so as to yield anamplified RF signal as an output.

In such a Doherty PA, both of the carrier and peaking PAs typicallyoperate when the input RF signal peaks. As input RF signal decreases inpower, the peaking PA typically turns off while the carrier PA operates.Such operating modes can result in efficient power amplification in manywireless applications.

SUMMARY

According to a number of implementations, the present disclosure relatesto a Doherty power amplifier (PA) that includes a carrier amplificationpath having an output that includes a carrier transformer, and a peakingamplification path having an output that includes a peaking transformer.The Doherty PA further includes a combiner configured to combine theoutputs of the carrier and peaking amplification paths into an outputnode. The combiner includes a quarter-wave circuit implemented betweenthe carrier and peaking transformers.

In some embodiments, the combiner can be a voltage combiner. Each of thecarrier and peaking transformers can include a primary loop and asecondary loop. Each of the carrier and peaking amplification paths caninclude a first PA cell and a second PA cell. The first and second PAcells can be configured as differential PA cells. Outputs of the firstand second PA cells of the respective amplification path can be coupledby the corresponding primary loop. The quarter-wave circuit can couplefirst ends of the secondary loops of the carrier and peakingtransformers. A second end of the secondary loop of the carriertransformer can be coupled to the output node. A second end of thesecondary loop of the peaking transformer can be coupled to a ground.

In some embodiments, each of the first and second PA cells can include aplurality of transistors arranged in a stack. In some embodiments, eachof the first and second PA cells can be implemented as a CMOS PA. Insome embodiments, the quarter-wave circuit can include an inductance Lhaving first and second ends coupled to the secondary loops of thecarrier and peaking transformers, respectively, a first capacitance C1implemented between the first end of L and a ground, and a secondcapacitance C2 implemented between the second end of L and the ground.C1 and C2 can have values that are substantially the same.

In some implementations, the present disclosure relates to aradio-frequency (RF) module having a packaging substrate configured toreceive a plurality of components, and a Doherty power amplifier (PA)implemented on the packaging substrate. The Doherty PA includes acarrier amplification path having an output that includes a carriertransformer and a peaking amplification path having an output thatincludes a peaking transformer. The Doherty PA further includes acombiner configured to combine the outputs of the carrier and peakingamplification paths into an output node. The combiner includes aquarter-wave circuit implemented between the carrier and peakingtransformers.

In some embodiments, the RF module can be a PA module. In someembodiments, the RF module can be a front-end module.

In some embodiments, at least some of the Doherty PA can be implementedon a CMOS die. The RF module can further include a bias circuitconfigured to provide bias signals to the carrier and peakingamplification paths.

In accordtance with some teachings, the present disclosure relates to awireless device having a transceiver configured to generate aradio-frequency (RF) signal, and a power amplifier (PA) systemconfigured to amplify the RF signal. The PA system includes a Doherty PAwith a carrier amplification path having an output that includes acarrier transformer and a peaking amplification path having an outputthat includes a peaking transformer. The Doherty PA further includes acombiner configured to combine the outputs of the carrier and peakingamplification paths into an output node. The combiner includes aquarter-wave circuit implemented between the carrier and peakingtransformers. The wireless device further includes an antenna incommunication with the output node of the Doherty PA and configured tofacilitate transmission of the amplified RF signal.

In some embodiments, the wireless device can be, for example, a cellularphone.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the inventions have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a Doherty power amplifier (PA) that canbe configured to include one or more features as described herein.

FIG. 2 shows a general configuration of a Doherty PA implemented toreceive an input radio-frequency (RF) signal and generate an amplifiedRF signal.

FIG. 3 shows an example of a Doherty PA where signals associated withcarrier and peaking PAs are combined with a current-combining approach.

FIG. 4 shows an example of a Doherty PA having one or more features asdescribed herein.

FIG. 5 shows that in some embodiments, a PA cell in the Doherty PA ofFIG. 4 can be implemented as a stack of transistors to sustain highvoltage swings.

FIG. 6 shows an example of the quarter-wavelength circuit of FIG. 4.

FIG. 7 shows an example of a simulated power-added efficiency (PAE)curve for the Doherty PA of FIG. 4.

FIG. 8 shows that in some embodiments, a Doherty PA having one or morefeatures as described herein can be implemented in a module.

FIG. 9 depicts an example wireless device having one or moreadvantageous features described herein.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

FIG. 1 shows a block diagram of a Doherty power amplifier (PA) 100 thatcan be configured to include one or more features as described herein.Such a PA can amplify the power of an input radio-frequency (RF) signal(RF_IN) to generate an amplified RF signal (RF_OUT) that can betransmitted.

As described herein, the Doherty PA 100 if FIG. 1 can be implemented asa voltage-combined Doherty CMOS PA. Although described in such a CMOScontext, it will be understood that one or more features of the presentdisclosure can also be implemented in other types of processtechnologies.

As described herein, signals processed through carrier and peaking PAscan be combined through a voltage combiner and a quarter wave-lengthcircuit at a secondary loop. Characteristic impedance of an impedancetransformation circuit can be approximately equal with an optimum loadimpedance (Ropt) for peaking and carrier PAs, and the carrier PA'sloadline can be increased to, for example, two times of Ropt when thepeaking PA is turned off at back-off power; therefore, the efficiencycan be enhanced accordingly. Examples of designs having such featuresare simulated in 0.18 μm SOI process, and the simulation resultsindicate that power-added efficiency (PAE) can be improved by 10% at 26dBm output power, when compared with a conventional Class AB PA at 1.85GHz.

It is noted that many wireless communication systems experienceincreases in information content, which typically leads to modulationsystems that need to operate with wider bandwidths and higher crestfactors. Doherty power amplification techniques are popular schemes forhigh-efficiency operation at a back-off power region. Doherty PAs canamplify modulated signals with high crest factors efficiently, sinceonly approximately half of a power cell is operated in a low powerregion.

It is also noted that integrating a Doherty architecture into a singlechip is typically challenging, since the architecture normally utilizesan input coupler and a quarter-wavelength transformer, which aretypically bulky. When implemented in a differential CMOS PA application,one typically needs to design a quarter wavelength transformationcircuits for each cell, and such an implementation can be difficult toachieve.

Described herein are examples related to a voltage combined Doherty PA,which can be implemented with a quarter wave-length transformationcircuit in the secondary loop of a transformer. Such a configuration canavoid complicated designs and be very suitable for voltage combinedtransformer-based PAs such as CMOS PAs.

FIG. 2 shows a general configuration of a Doherty PA 10 implemented toreceive an input RF signal (RF_in) and generate an amplified RF signal(RF_out). The input RF signal is shown to be split into a carrieramplification path 12 and a peaking amplification path 14. Processed RFsignals from such amplification paths are shown to be combined so as toyield RF_out.

FIG. 3 shows an example of a Doherty PA 10, where signals associatedwith the carrier and peaking PAs are combined with a current-combiningapproach. For example, the carrier amplification path can include acarrier PA 20 followed by a quarter-wavelength of a transformationcircuit 22; and the peaking amplification path can include a phasecompensation circuit 24 followed by a peaking PA 26. The combined outputof the carrier and peaking amplification paths is shown to be coupled toa load having an optimum impedance (Z_(L)=R_(Opt)).

Configured in the foregoing manner, each amplification path presentsapproximately half of the load impedance (Z_(L)) at the common node. Forexample, the carrier amplification path presents an impedance ofZ_(C)=0.5R_(Opt), and the peaking amplification path presents animpedance of Z_(P)=0.5R_(Opt). Therefore, back-off efficiency may not beimproved significantly in cellular PA applications. For example, whenimplemented as CMOS PAs, numerous transformation circuits in the currentcombining approach can cause amplitude and phase imbalance anddeteriorate the efficiency and linearity.

FIG. 4 shows an example of a Doherty PA 100 where each of the carrier PAand peaking PA can include differential PA cells. For example, thecarrier PA is shown to include PA cells 112 a, 112 b, and the outputs ofsuch PA cells are shown to be coupled by a primary loop of a carriertransformer 114. Similarly, the peaking PA is shown to include PA cells122 a, 122 b, and the outputs of such PA cells are shown to be coupledby a primary loop of a peaking transformer 124.

A secondary loop of the carrier transformer 114 is shown to be coupledwith a secondary loop of the peaking transformer 124 through aquarter-wavelength circuit 130. In the example of FIG. 4, such aquarter-wavelength circuit is shown to be implemented between node A (ofthe carrier transformer 114) and node B (of the peaking transformer124). Associated with node A are voltage V_(A) and current I_(A).Similarly, voltage V_(B) and I_(B) are associated with node B.

In the example of FIG. 4, node A and an output node 116 can form the twoends of the secondary loop of the carrier transformer 114. Node B and aground node 126 can form the two ends of the secondary loop of thepeaking transformer 124. In some embodiments, the carrier PA can beimplemented closer to the RF output node 116 than the peaking PA.Accordingly, the peaking PA can be implemented closer to the ground node126 than the carrier PA.

Referring to FIG. 4, it is noted that a load impedance seen by carrier(Z_(c)) and peaking (Z_(p)) PA can be represented as follows:

$\begin{matrix}{{Z_{c} = {\frac{Z_{T}^{2}}{R_{L}}\frac{V_{out}}{V_{out} - V_{B}}}},} & (1) \\{Z_{p} = {\frac{V_{B}}{V_{out}}{Z_{L}.}}} & (2)\end{matrix}$

where Z_(T) is the characteristic impedance of the quarter-wavelengthcircuit 130, and R_(L) is the load impedance at the output of secondaryloop. Hence, if V_(B) is changed with the output power, the loadimpedance seen by carrier PA can be modulated dynamically and behave asDoherty PA performance.

Assuming the carrier PA operates at Class A bias and the peaking PAoperates at Class B bias, each PA's optimum load impedance can beR_(opt). Accordingly, I_(max) can be a maximum current at saturatedpower, and V_(max) can be a maximum voltage swing of each PA.

Referring to FIG. 4, it is further noted that the currents at node A andB can be express as follows:

$\begin{matrix}{{I_{A} = {\frac{1}{4}\left( {1 + \alpha} \right)I_{\max}}},} & (3) \\{I_{B} = {\frac{1}{2}\alpha \; {I_{\max}.}}} & (4)\end{matrix}$

where α can vary between 0 and 1. When the peaking PA is turned off,α=0; and when the peaking PA is PA is turned on, α=1.

A relationship of current and voltage at nodes A and B can be expressedas follows, in terms of quarter-wavelength load impedancetransformation:

$\begin{matrix}{{\frac{V_{A}}{I_{A}}\frac{V_{B}}{I_{B}}} = {Z_{T}^{2}.}} & (5)\end{matrix}$

Assuming that power is generally constant at nodes A and B(V_(A)I_(A)=V_(B)I_(B)), Equation 5 can be simplifies as

$\begin{matrix}{Z_{T} = {\frac{V_{B}}{I_{A}} = {\frac{V_{A}}{I_{B}}.}}} & (6)\end{matrix}$

Hence, the carrier and peaking PAs' load line impedances can beexpressed as:

$\begin{matrix}{{Z_{c} = {\frac{V_{out} - V_{A}}{V_{out}}Z_{L}}},} & (7) \\{Z_{p} = {\frac{V_{B}}{V_{out}}{Z_{L}.}}} & (8)\end{matrix}$

Accordingly, the carrier PA's voltage swing can be expressed as:

$\begin{matrix}\begin{matrix}{V_{c} = {Z_{c}I_{A}}} \\{= {\frac{V_{out} - V_{A}}{V_{out}}Z_{L}\frac{1}{4}\left( {1 + \alpha} \right)I_{\max}}} \\{= {\frac{V_{out} - {Z_{T}I_{B}}}{V_{out}}Z_{L}\frac{1}{4}\left( {1 + \alpha} \right)I_{\max}}} \\{= {\left( {1 - {\frac{Z_{T}}{V_{\max}}\frac{1}{2}\alpha}} \right)Z_{L}\frac{1}{4}\left( {1 + \alpha} \right){I_{\max}.}}}\end{matrix} & (9)\end{matrix}$

It is noted that when carrier and peaking amplifications operate atoptimum load impedance matching, Z_(L)=2R_(opt), with

$R_{opt} = \frac{V_{\max}}{I_{\max}}$

for Class A bias operation. The voltage swing of the carrier PA can beexpressed further as:

$\begin{matrix}{V_{c} = {I_{\max}{{R_{opt}\left( {1 + \alpha - {\frac{Z_{T}}{2\; R_{opt}}\alpha} - {\frac{Z_{T}}{2\; R_{opt}}\alpha^{2}}} \right)}.}}} & (10)\end{matrix}$

From the foregoing examples, one can observe that if V_(c) can besubstantially maintained with high voltage swing, efficiency can beimproved at back-off power. Assuming or setting a condition wherediff(V₁₂)=0, one can express ZT as:

Z_(T)=R_(opt).   (11)

From the foregoing examples, one can see that the carrier and peakingPAs will see R_(opt) at maximum power when both of the carrier andpeaking PAs are turned on, and carrier PA will see 2R_(opt) when thepeaking PA is turned off at, for example, 6 dB back-off power.Accordingly, efficiency of the Doherty PA 100 can be improved.

In some embodiments, the Doherty PA 100 of FIG. 4 can be implemented ina number of process technologies, including silicon-on-insulator (SOI)process technology. For example, the PA cells 112 a, 112 b, 122 a, 122 bcan be implemented in SOI 0.18 μm process to operate at one or morefrequencies or frequency ranges (e.g., at 1.85 GHz). It will beunderstood that one or more features of the present disclosure can beimplemented with other process technologies, dimensions, and/oroperating frequencies.

FIG. 5 shows that in some embodiments, each of the PA cells (112 a, 112b, 122 a or 122 b) can be implemented as, for example, a three-stackthick transistors to sustain high voltage swings at high load-lines. Inanother example, a total gate width of 18 μm can be supplied withapproximately 3.4V. It will be understood that one or more features ofthe present disclosure can be implemented with stack sizes, gatedimensions, and/or operating voltages.

In some embodiments, the carrier PA can be biased at light Class AB, andthe peaking can be biased at deep Class AB (e.g., more close to ClassB). It will be understood that other biasing configurations can also beimplemented.

FIG. 6 shows that in some embodiments, the quarter-wavelength circuit130 of FIG. 4 can be implemented as a pi-network, with an inductance Lbetween the nodes A and B, and a capacitance (C1 or C2) that coupleseach of the nodes A and B to ground. For the foregoing examples ofdesign and/or operating parameters, the inductance L can be an inductorhaving a value of approximately 4 nH, and each of the capacitances C1,C2 can be a lumped capacitor having a value of approximately 9 pF. Itwill be understood that other values of L, C1 and/or C2 can beimplemented, depending on designs. It will also be understood that otherquarter-wavelength circuits can be implemented. Further, althoughvarious examples are described herein in the context of aquarter-wavelength coupling between the carrier and peakingtransformers, it will be understood that one or more features of thepresent disclosure can also be implemented in other wavelength relatedcouplings.

FIG. 7 shows an example of simulated PAE curves for the Doherty PA 100of FIGS. 4-6 (“DPA” curve), and for a conventional Class AB PA(“Non-DPA” curve). One can see that at about 26 dBm output power, theDPA can achieve a PAE of about 35%, which is an improvement of about 10%over the Non-DPA PAE value.

FIG. 8 shows that in some embodiments, a Doherty PA having one or morefeatures as described herein can be implemented in a module such as a PAmodule (PAM) or a front-end module (FEM). In the example of FIG. 8, amodule 300 is shown to include a Doherty PA 100 having one or morefeatures as described herein. In some embodiments, such a PA can beimplemented in a PA die 304. Such a die can be, for example, a CMOS die.

In the example of FIG. 8, a biasing circuit 314 is depicted as beingimplemented on a separate die 312. However, it will be understood thatthe PA 100 and the biasing circuit 314 can be configured in othermanners (e.g., on a common die).

In the example module 300 of FIG. 8, the die 304, 312 are shown to bemounted on a packaging substrate 302. The die 304 can include aplurality of electrical contact pads 306 configured to allow formationof electrical connections 308 such as wirebonds between the die 304 andcontact pads 310 formed on the packaging substrate 302. Similarly, thedie 312 can include a plurality of electrical contact pads 316configured to allow formation of electrical connections 318 such aswirebonds between the die 312 and contact pads 320 formed on thepackaging substrate 302.

The packaging substrate 302 can be configured to receive a plurality ofcomponents such as the die 304, 312 and one or more SMDs (e.g., 320). Insome embodiments, the packaging substrate 302 can include, for example,a laminate substrate.

In some embodiments, the module 300 can also include one or morepackaging structures to, for example, provide protection and facilitateeasier handling of the module 300. Such a packaging structure caninclude an overmold formed over the packaging substrate 302 anddimensioned to substantially encapsulate the various circuits andcomponents thereon.

It will be understood that although the module 300 is described in thecontext of wirebond-based electrical connections, one or more featuresof the present disclosure can also be implemented in other packagingconfigurations, including flip-chip configurations.

In some implementations, a device and/or a circuit having one or morefeatures described herein can be included in an RF device such as awireless device. Such a device and/or a circuit can be implementeddirectly in the wireless device, in a modular form as described herein,or in some combination thereof. In some embodiments, such a wirelessdevice can include, for example, a cellular phone, a smart-phone, ahand-held wireless device with or without phone functionality, awireless tablet, etc.

FIG. 9 depicts an example wireless device 400 having one or moreadvantageous features described herein. In the example of FIG. 9, one ormore PAs 100 can include one or more features as described herein. SuchPAs can facilitate, for example, multi-band operation of the wirelessdevice 400. In embodiments where the PAs are packaged into a module,such a module can be represented by a dashed box 300.

The PAs 100 can receive their respective RF signals from a transceiver410 that can be configured and operated to generate RF signals to beamplified and transmitted, and to process received signals. Thetransceiver 410 is shown to interact with a baseband sub-system 408 thatis configured to provide conversion between data and/or voice signalssuitable for a user and RF signals suitable for the transceiver 410. Thetransceiver 410 is also shown to be connected to a power managementcomponent 406 that is configured to manage power for the operation ofthe wireless device 400. Such power management can also controloperations of other parts of the module 300.

The baseband sub-system 408 is shown to be connected to a user interface402 to facilitate various input and output of voice and/or data providedto and received from the user. The baseband sub-system 408 can also beconnected to a memory 404 that is configured to store data and/orinstructions to facilitate the operation of the wireless device 400,and/or to provide storage of information for the user.

In the example wireless device 400, outputs of the PAs 100 are shown tobe matched (via match circuits 420) and routed to an antenna 416 viatheir respective duplexers 412 a-412 d and a band-selection switch 414.The band-selection switch 414 can be configured to allow selection of anoperating band. In some embodiments, each duplexer 412 can allowtransmit and receive operations to be performed simultaneously using acommon antenna (e.g., 416). In FIG. 9, received signals are shown to berouted to “Rx” paths (partially shown) that can include, for example, alow-noise amplifier (LNA).

A number of other wireless device configurations can utilize one or morefeatures described herein. For example, a wireless device does not needto be a multi-band device. In another example, a wireless device caninclude additional antennas such as diversity antenna, and additionalconnectivity features such as Wi-Fi, Bluetooth, and GPS.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Additionally, the words “herein,” “above,” “below,” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Wherethe context permits, words in the above Description using the singularor plural number may also include the plural or singular numberrespectively. The word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list, and anycombination of the items in the list.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While some embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A Doherty power amplifier (PA) comprising: acarrier amplification path having an output that includes a carriertransformer; a peaking amplification path having an output that includesa peaking transformer; and a combiner configured to combine the outputsof the carrier and peaking amplification paths into an output node, thecombiner including a quarter-wave circuit implemented between thecarrier and peaking transformers.
 2. The Doherty PA of claim 1 whereinthe combiner is a voltage combiner.
 3. The Doherty PA of claim 2 whereineach of the carrier and peaking transformers includes a primary loop anda secondary loop.
 4. The Doherty PA of claim 3 wherein each of thecarrier and peaking amplification paths includes a first PA cell and asecond PA cell.
 5. The Doherty PA of claim 4 wherein the first andsecond PA cells are configured as differential PA cells.
 6. The DohertyPA of claim 5 wherein outputs of the first and second PA cells of therespective amplification path are coupled by the corresponding primaryloop.
 7. The Doherty PA of claim 6 wherein the quarter-wave circuitcouples first ends of the secondary loops of the carrier and peakingtransformers.
 8. The Doherty PA of claim 7 wherein a second end of thesecondary loop of the carrier transformer is coupled to the output node.9. The Doherty PA of claim 8 wherein a second end of the secondary loopof the peaking transformer is coupled to a ground.
 10. The Doherty PA ofclaim 4 wherein each of the first and second PA cells includes aplurality of transistors arranged in a stack.
 11. The Doherty PA ofclaim 4 wherein each of the first and second PA cells is implemented asa CMOS PA.
 12. The Doherty PA of claim 4 wherein the quarter-wavecircuit includes an inductance L having first and second ends coupled tothe secondary loops of the carrier and peaking transformers,respectively, a first capacitance C1 implemented between the first endof L and a ground, and a second capacitance C2 implemented between thesecond end of L and the ground.
 13. The Doherty PA of claim 12 whereinC1 and C2 have values that are substantially the same.
 14. Aradio-frequency (RF) module comprising: a packaging substrate configuredto receive a plurality of components; and a Doherty power amplifier (PA)implemented on the packaging substrate, the Doherty PA including acarrier amplification path having an output that includes a carriertransformer and a peaking amplification path having an output thatincludes a peaking transformer, the Doherty PA further including acombiner configured to combine the outputs of the carrier and peakingamplification paths into an output node, the combiner including aquarter-wave circuit implemented between the carrier and peakingtransformers.
 15. The RF module of claim 14 wherein the RF module is aPA module.
 16. The RF module of claim 14 wherein the RF module is afront-end module.
 17. The RF module of claim 14 wherein at least some ofthe Doherty PA is implemented on a CMOS die.
 18. The RF module of claim17 further comprising a bias circuit configured to provide bias signalsto the carrier and peaking amplification paths.
 19. A wireless devicecomprising: a transceiver configured to generate a radio-frequency (RF)signal; a power amplifier (PA) system configured to amplify the RFsignal, the PA system including a Doherty PA with a carrieramplification path having an output that includes a carrier transformerand a peaking amplification path having an output that includes apeaking transformer, the Doherty PA further including a combinerconfigured to combine the outputs of the carrier and peakingamplification paths into an output node, the combiner including aquarter-wave circuit implemented between the carrier and peakingtransformers; and an antenna in communication with the output node ofthe Doherty PA and configured to facilitate transmission of theamplified RF signal.
 20. The wireless device of claim 19 wherein thewireless device includes a cellular phone.